In an electrophotographic copier to which the Calson Method is applied, image formation is conducted as follows. After a photoreceptor has been uniformly charged, the photoreceptor is subjected to image exposure so that the electric charge on the photoreceptor is erased image-wise and an electrostatic latent image is formed. This electrostatic latent image is developed by toner, and the obtained toner image is transferred and fixed onto a transfer sheet such as a sheet of paper.
After the toner image has been transferred onto the transfer sheet, residual toner is removed and further static electricity is discharged from the photoreceptor surface. In this way, the photoreceptor surface is purified. Accordingly, the photoreceptor for electrophotographic use must be provided with an appropriate charging characteristic and high sensitivity, and further dark decay of the photoreceptor must be low. Furthermore, the photoreceptor is required to have physical properties such as an anti-rubbing property, anti-abrasion property and anti-damage property since it is repeatedly used. Besides it is required that the photoreceptor is resistant to ozone generated in the process of corona discharge and also resistant to ultraviolet rays incident on the photoreceptor in the process of exposure.
Since copiers have come into wide use recently, there is a demand for a high speed type copier capable of processing a large number of copies at high speed and a compact type copier for office or family use.
In order to meet the demand of users, it is necessary to develop a highly sensitive and durable photoreceptor.
As an example of the highly sensitive and durable photoreceptor, an inorganic photoreceptor is known, the photosensitive layer of which primarily contains an inorganic photoconductive substance of selenium type. However, the inorganic photoconductive substance of selenium type is harmful to the human body. Besides, it is difficult of machine the inorganic photoconductive substance of selenium type, so that the productivity is not high and further the moisture-resistant property is low.
Recently, research and development are actively performed so as to provide an organic photoreceptor having high productivity and moisture-resistant property and causing no public pollution. Attention is given to functionary separated multi layered type photoreceptor in which a function of charge generation and a function of charge transport are performed by different substances, and a substance to perform each function is selected from a wide range in accordance with the desired characteristics.
In the process of research and development, many results are proposed with respect to the charge generation and charge transport substances having charge generation and charge transport functions.
For example, an azo pigment effectively used as a charge generation substance is disclosed in Japanese Patent Publication Open to Public Inspection No. 222152/1983, and a styryl compound effectively used as a charge transport substance is disclosed in Japanese Patent Publication Open to Public Inspection No. 149652/1988.
As described above, an organic photoreceptor, the sensitivity and durability of which are equal or superior to the selenium photoreceptor, has been developed and put into practical use.
In accordance with the enhancement of sensitivity and durability of the organic photoreceptor, there is a demand for extending the use of the organic photoreceptor from a low or medium speed machine, the processing speed of which is not more than 40 sheets/min, to a high speed machine, the processing speed of which is not less than 50 sheets/min when the sheets of size A4 are processed. In this case, the high speed machine includes a slit exposure type high speed copier in which a photoreceptor drum is used, and a flash exposure type high speed copier in which a photoreceptor belt is used.
However, compared with the low and medium speed machines described above, the high speed machine must be operated in a severe image forming condition. Especially, it is necessary to overcome the following problems relating to image exposure.
In the case of copying at high speed, a surface of the photoreceptor is exposed to light of high intensity over a short period of time. Accordingly, due to the reciprocity law failure, various problems are caused such as a deterioration of the sensitivity, increase of the residual potential and a defective formation of the latent image. Therefore, fog is caused and the formed image is blurred.
When copying is conducted in the flash exposure type copier at high speed, it is necessary to increase the moving speed of the photoreceptor. Accordingly, time of exposure of the photoreceptor conducted by the slit becomes short, and the same problems as those of flash exposure are caused. In order to make the copier compact, a diameter of the photoreceptor drum is reduced. Therefore, in order to obtain an image having a predetermined resolution, it is necessary to reduce the slit width through which image exposure is conducted. For this reason, it is required to conduct exposing over a short period of time using light of high intensity. Therefore, the slit exposure is affected by the reciprocity law failure in the same manner as the flash exposure.
In view of the above circumstances, the present invention has been accomplished. An object of the present invention is to provide an image forming method to be applied to a flash exposure type and a slit exposure type high speed copier characterized in that: a defective image which is due to insufficient sensitivity is not caused; and a defective image which is due to a decrease in the sensitivity and an increase in the residual potential is not caused even when the high speed copier is repeatedly operated.